ocean on Europa

Cutaway view of the possible internal structure of
Europa. A metallic (iron, nickel) core (gray) is drawn to the correct
relative size. This is surrounded by a rock shell (brown), which in
turn is surrounded by a shell of water in ice or liquid form (blue
and white). The surface layer of Europa is shown as white to indicate
that it may differ from the underlying layers

Artist's conception of an underground ocean on Europa.
If the moon's heat – possibly from volcanic activity in its
rocky mantle – is intense enough and the ice shell is thin enough,
the ice shell can directly melt, causing regions of what appear to
be broken, rotated and tilted ice blocks on the surface. Credit: Michael
Carroll/NASA/JPL

Great interest surrounds the likelihood that an ocean of liquid water exists
beneath the icy surface of Europa. This ocean
may be as much as 150 km deep, which would mean it contained twice as much
water as exists in all of Earth's oceans.

The history of this idea can be traced back to a 1971 theoretical paper
in which the author, the American astronomer John S. Lewis, argued that
" ... the Galilean satellites of Jupiter and the large satellites of Saturn, Uranus,
and Neptune very likely have extensively
melted interiors."

Speculation about a sub-ice ocean and hypothetical Europan life (see Europa,
life on) stepped up following Voyager 2's
close encounter with Europa in 1979,1, 2 while by the time the Galileo spacecraft arrived at Jupiter in
1995, the ocean model was well established as a plausible hypothesis. What
the new probe found only strengthened its case. High-resolution images provided
clear evidence of near-surface melting and the movements of large blocks
of icy crust, similar to those of icebergs or ice rafts on Earth.3 Galileo's cameras also revealed very few impact craters – a sure sign
that resurfacing has taken place in geologically recent times, no more than
30 million years ago. Various sources of heat have been discussed by planetary
astronomers over the years as the means by which Europa's ice shelf might
be kept molten from below, but the principal mechanisms are now thought
to be tidal distortions caused by the shifting gravitational pulls of Jupiter
and the other large Jovian moons and the internal decay of radioactive elements.

Similar tidal heating may have created
an ocean on Callisto, drives volcanoes
on Io, and may give rise to water geysers on
Europa, though none have yet been observed.